State of the Art
The coating systems for excessive application can use applicator rollers (DE C 36 23 402) and applicator chambers which are open towards the web (EP-C 0 051 698). The doctor elements are usually mechanical doctor elements, such as scraping blades (DE-C 36 23 402), wiper rods (DE-C 30 22 955), wiper bars (EP-C 0 109 520) and so-called air knives, which wipe off the excess with compressed air coming from a slotted nozzle (DE-A 40 14 463). In order to achieve higher production speeds, i.e. higher speeds of the web of material, in these known processes the web of material is guided around a backing roller while the coating material is applied in excess and while the excess is wiped off, and is therefore supported by the roller over its entire width during the coating process.
Practice has proven that with the coating using the known application and doctoring systems, it is very difficult to obtain an end product with an even, and therefore high-quality coating, when working with very high production speeds of over 1000 m/min. At high web speeds, irregularities occur in the finished coating, such as variations in the coating weight, stripes or streaks.